1. Field of the Invention
The present invention relates in general to an image reproducing system utilizing a so-called "minimized average error" technique in quantization of multilevel original intensity values at picture elements of a source image. More particularly, this invention is concerned with improvements in image reproducing apparatus and process wherein the original intensity value at each of the picture elements (usually called "cells" or "pels") is divided into a plurality of fractions corresponding to divisions of the maximum value of the intensity value, and wherein the minimized average error technique is practiced for each of those fractions.
2. Discussion of the Related Art
In the field of image reproduction, an image processing technique is known for providing a gray scale or "halftone" picture on a bilevel display or other output device, with one of two levels ("on" and "off"; "black" and "white"; or "bright" and "dark"), in which a multilevel input image signal is compared with a threshold value. That is, the multilevel input image signal is quantized into a bilevel output image signal, using the suitably determined threshold value. For improved reproduced image quality, the threshold value used for each picture element under consideration is determined by either the original intensity value alone at that picture element under consideration, or both this original intensity value and the previously determined intensity values at the picture elements in the neighborhood of the picture element under consideration.
The well known "minimized average error" technique is a typical example of the latter method in which the intensity values at the neighboring picture elements are also considered in determining the intensity value at the picture element in question. This technique was based on a technique disclosed by M. R. Schroeder, Bell Telephone Laboratories, Inc., in his article "Images from computers", IEEE SPECTRUM, March 1969, which refers to "local spatial averages of brightness". The "brightness" correspond to the "intensity" indicated above. Some years later, J. F. Jarvis et al. improved the technique proposed by M. R. Schroeder, as "minimized average error" technique or approach, which is disclosed in "A Survey of Techniques for the Display of Continuous Tone Pictures on Bilevel Displays", COMPUTER GRAPHICS AND IMAGE PROCESSING 5, 13-40 (1976), BELL LABORATORIES. The "minimized average error approach" is to choose successive values Pxy of the displayed intensity in such a way as to minimize the accumulated error between the original and display images.
Described in detail by reference to FIG. 3 wherein a picture element under consideration whose intensity value is to be determined is indicated by a hatched square block, intensity error values E(k, 1) at the neighboring picture elements whose intensity values have been already determined are used to determine or correct the intensity value at the picture element under consideration. The intensity error values E(k, l) are utilized in the form of a weighted accumulated average A(m, n) which is obtained by using an appropriate weighting matrix W as illustrated in FIG. 4, according to the following equation (1): EQU A(m, n)=(1/.SIGMA.W(k, l))..SIGMA.W(k, l).E(k, l) (1)
Using the weighted average A(m, n) of the intensity error values E(k, l), a corrected image intensity value P(m, n) at the picture element under consideration is obtained according to the following equation (2): EQU P(m, n)=A(m, n)+I(m, n) (2)
where, I(m, n): non-corrected or original image intensity value
The corrected image intensity value P(m, n) is compared with a predetermined threshold value, which is usually equal to D/2, to determine whether an image dot should be placed at the picture element under consideration, namely, whether that picture element should be turned "on". "D" represents the maximum value possible of the original intensity I(m, n). If the maximum value R is equal to "1", the threshold value is equal to "0.5". In this case, if P(m, n).gtoreq.0.5, then the picture element is turned "on" else the picture element is turned "off".
The intensity error at the picture element under consideration is determined according to the following equations (2) and (3): EQU If P(m, n).gtoreq.0.5, then E(m, n)=P(m, n)-1 (2) EQU If P(m, n).ltoreq.0.5, then E(m, n)=P(m, n) (3)
The above procedure is repeated for all the picture elements corresponding to the source image or original image, to determine whether the individual picture elements should be turned "on" or "off", to reproduce the source image on an appropriate output device such as a display device or a printer.
The principle of the "minimized average error" technique, which has been described above, is effectively applicable to a bilevel output device which has two output levels at each picture element. On the other hand, some output devices are known, which are adapted to reproduce the source image in one of three output levels at each picture element according to a multilevel output image signal, by means of pulse width modulation or power amplitude modulation. For instance, a printer is available capable of reproducing the source image, with three to sixteen output levels at each picture element. However, the minimized average error technique has not been suitably applicable to such multilevel output device. In view of this drawback, an improvement is proposed as disclosed in JP-A-3-18177 (Laid-open Publication of unexamined Japanese Patent Application published in 1991), wherein two or more threshold values (e.g., three threshold values) are used to quantize a multilevel input image signal into a multilevel output image signal (e.g., eight output levels), following minimized average error approach.
Like the bilevel image reproducing system wherein the bilevel output image signal is used, however, the multilevel image reproducing system suffers from an undesirable dummy image areas or blur in the reproduced image, which deteriorate the image reproduction quality, as experienced on the conventional image reproducing systems using the minimized average error technique.